How to Prevent Critical Failures of Evaporative Cooling Equipment
Systematic and thorough attention to the fan and motor assemblies is essential to avoid the unexpected interruption of service.
Most catastrophic system failures are easily avoidable because they are linked to a negligent lack of maintenance. Consider a catastrophic fan failure at a Midwestern manufacturing plant that occurred on a Friday afternoon, grinding the weekend process to a halt and incurring labor to be billed out at weekend rates.
A facility engineer noticed that water temperatures in the 1,200-ton tower were not decreasing sufficiently, yet there were no alarms. A maintenance technician was sent to investigate the tower and found that the fan assembly had blown apart. The fan screens, cylinder, mechanical supports and other smaller components were damaged as well.
Closer inspection revealed that the fan shaft had significant play due to worn-out bearings, causing the fan blades to repeatedly hit the cylinder. Lacking vibration controls, the entire assembly eventually blew apart, destroying most of the fan section in the process. Had the tower been subject to routine inspection and maintenance, this expensive problem — with significant downtime consequences — could have been avoided. Not counting lost production, the owner spent more than $20,000 in parts and service due to inadequate maintenance and monitoring of fan bearings.
Evaporative cooling equipment — like any mechanical unit of machinery — requires preventive maintenance to maximize its utility. Simple maintenance procedures provide enormous benefits to connected systems and equipment, manufacturing processes and plant personnel. Benefits include the reduction of operating costs, downtime and capital expenses as well as improved system efficiency and increased equipment lifespan. In addition, keep in mind that repair costs often pale in comparison to costs that stem from the downtime loss of production.
Central to each of these benefits is the prevention of critical or catastrophic failures. Most failures of evaporative cooling equipment involve moving components. They typically can be separated into three specific parts or systems:
- Fan motor.
- Power transmission components such as bearings, sheaves and belts.
Evaporative cooling equipment manufacturers provide resources such as the operations and maintenance (O&M) instructions and maintenance schedules to assist facility personnel. These tools often focus on the basic level of maintenance required for proper operation. In addition, a range of innovative processes and products can provide a greater level of security for the owner or facility manager.
The Fan Motor: The Heart of the System
The fan motor is the heart of the evaporative cooling equipment. Like the human heart, it requires extra care for the well-being of the equipment.
Control sequencing is an appropriate starting point. When applied to across-the-line starts rather than the application of variable-frequency drive (VFD), it is critical to limit start-stop sequences to no more than six per hour. Each motor start is associated with a spike in voltage — referred to as in-rush current — that increases the motor operating temperature and can lead to motor winding insulation failure. Limiting the occurrence of these events protects the motor. Increasing the temperature control deadband or lowering the setpoint are two ways to reduce fan-cycling frequency.
VFDs are the preferred method for control of fan motors. However, there are still some concerns. Chief among them is the issue of stray currents. VFDs simulate a sine wave via on-off switching of current. Byproducts of this method are stray currents that often ground themselves at the motor bearings. Failure to address this will result in premature motor failure. A simple and cost-effective solution is available in the form of shaft-grounding rings. They operate much like a lightning rod and provide a ground for the stray currents outside of the motor housing.
Moisture intrusion is another common mode of failure and can be a serious challenge. Evaporative cooling equipment must operate, by necessity, in a humid environment. That is why fan motors are designed to function properly with some influence by moisture. An unwanted side effect is that they are susceptible to moisture contamination, often from condensation. A key remedy is to ensure that condensate drains are positioned at the lowest point of the motor mounting or enclosure.
When applied to motors, the term “totally enclosed” often leads to a false sense of security. The type of motor used for these systems is designed to prevent the required cooling air from flowing over the motor windings, but it is not airtight.
A proactive approach — one that greatly limits the possibility of moisture contamination — is the installation of space heaters within the motor housing. Space heaters are wired to the motor control panel so that they are energized when the motor shuts off and de-energized when the motor turns on. This helps to maintain a consistent internal temperature, preventing condensation.
Causes of Fan Failure in Evaporative Cooling Systems
While fans are much less complex than the motor, their maintenance is no less critical. Fan failures are chiefly associated with vibration, balance or structural impairments. As a result, routine inspection and monitoring are required.
Vibration cut-out switches — designed to protect the fan in the event of excessive vibration — provide a fairly reliable “insurance policy” against fan failures. However, more aggressive approaches will improve equipment dependability. After all, by the time a fan is vibrating excessively, damage is occurring — or has already occurred — to components such as fan bearings.
A smarter approach to ensuring fan health comes with the installation of vibration sensors, which generate control signals proportional to vibration levels. These devices allow constant monitoring of the drive system and can pinpoint degradation of components long before a failure occurs.
Another key facet of fan maintenance is fan balancing. Proper balance is critical to unit operation and performance. Fan balancing can be either static or dynamic. Static balancing is performed when the fan is at rest. Fan suppliers routinely balance fans, statically, following manufacture.
Dynamic balancing, however, is performed while the fan is in motion. Dynamic balance values often differ from static balance values because the fan is subjected to the influence of moving components such as the sheaves and belts. Any time the drive system is disrupted, the fan system should be dynamically balanced. Disruptions take the form of component replacements such as a fan, sheave, shaft or a set of bearings. This will ensure proper operation and increased reliability and longevity.
Fan blades and fan surfaces should be routinely inspected. Corrosion, cracks, gouges and, sometimes, ice formation may contribute to fan imbalance, leading to premature or catastrophic fan failure. Any corroded surfaces should be sanded and treated with an industrial-grade rust inhibitor. Cracks and gouges often result from blade contact with foreign objects. The objects do not require a great deal of size or density, given that fans often rotate upwards of 400 rpm. Problems like these should be reported to a service company and followed up with a complete unit inspection.
Reasons Fan-Drive Components Fail
In between the fan motor and the fan are the drive components. Whether gear driven or belt driven, proper alignment is critical. Gear drives require a coupling between the motor and gear. These can be close couplings or a floating shaft.
Not only can misalignment contribute to a premature failure, but the couplings themselves also are under heavy and frequent torque conditions. Because of this, the hardware and flex elements are designed to absorb torque and usually will be the first to show signs of wear. Regular inspection and timely replacement of these components involve considerably less cost and downtime than a full gear-drive or fan failure.
Alignment of belt-driven systems is just as important. Part replacement of belt-driven systems is often less costly and requires less downtime than gear-driven systems. Sheave alignment should result in a four-point contact of a straight edge or laser tool, with a maximum deviation of 0.0625”. The maximum mid-point deflection of the belt should be 0.5”.
Fan-shaft bearings often are the most frequently replaced part of evaporative cooling equipment. Replacements can be reduced with increased care and attention to lubrication. For starters, use only greases that are approved by equipment manufacturers. All grease formulations contain some form of detergent, and it is these chemicals that do not play well together when mixed. Consistency plays an important role. If new grease is introduced, it is critical that the entire system — including bearings and lube lines — be fully purged of the old lubricant.
Lubrication schedules typically are provided by equipment manufacturers. These are based on long-term service data and are not to be ignored. Just as important is the manner by which bearings are regreased. Grease often is applied too vigorously, causing bearing seals to blow out, which is sure to cause premature failure.
Automatic bearing greasers can be a real advantage. Many are designed to provide a timed release of lubrication. The discharge is performed under low pressure to prevent damage to bearing seals. Despite these advantages, automatic bearing greasers should not be considered a substitute for visual inspections.